I don't like the polling/interrupt setup part:
- for a nic driver, there is no irq line that could be requested by
mii_phy_irq_enable().
- if the mii bus driver uses it's own timers, then locking within the
nics will be more difficult.

I'm not sure I accept the argument that locking will be more difficult.
Jason's patch requires that a callback be registered for the interrupt
or the polling (My update has a similar scheme). The function you
register is essentially like an extra interrupt, except it is never
invoked at interrupt time. All the function has to do is react
properly to link state. If, previously, you checked link state in your
interrupt handler, you could still do it there, I suspect.

Could you make that part optional? For a nic driver, I would prefer if
I could just call the ->startup part without the request_irq. If the
nic irq handler notices that the nic got an event, then it would call
an appropriate mii_bus function.

I think it would be doable to arrange the interface such that drivers
could adopt only the PHY configuration infrastructure, and not any of
the polling/interrupt infrastructure. Of course, as it is, it is at
least WHOLLY optional, so no driver has to use it at all.

This also applies for something like /dev/phy/xy: With natsemi, it
would be very tricky to add proper locking. The nic as an internal phy
and an external mii bus. The internal phy is partially visible on the
external bus and any accesses to the phy id of the internal phy on the
external bus cause lockups. No big deal, I just move the internal phy
around [the phy id doesn't matter], but I would prefer if I have to do
that just for ethtool, not for multiple interfaces.

I agree with this point -- Accessing the PHY through /dev registers is
a recipe for some mess. Though I could be convinced that it is
manageable. I do think, however, that the ethtool interface is
sufficient to the task.